The incorporation of an electronic trip circuit within the covers of molded case circuit breakers has resulted in a compact arrangement of the internal circuit breaker operating components. The positioning of the trip unit printed wire board within the circuit breaker cover next to the circuit breaker operating handle requires that the operating mechanism open and close the circuit breaker contacts with a minimum stroke of the externally accessible circuit breaker operating handle. U.S. Pat. No. 4,945,443 describes one such circuit breaker having an electronic trip unit within the circuit breaker. The electrical parameters are visually accessed by means of a keypad and display arranged on the exterior surface of the circuit breaker cover.
A compact circuit breaker operating mechanism is described within U.S. Pat. No. 4,782,583 entitled "Method of Assembling a Molded Case Circuit Breaker Crossbar", which operating mechanism interconnects between the externally accessible circuit breaker operating handle and the movable contact arm by means of an operating cam assembly.
U.S. patent application Ser. No. 330,521 filed Mar. 30, 1989 entitled "Molded Case Circuit Breaker Movable Contact Arm Arrangement" describes a molded case circuit breaker using an electronic trip unit to articulate the circuit breaker operating mechanism to separate the circuit breaker contacts upon the occurrence of an overcurrent condition of predetermined magnitude and duration. The current limiting feature of the circuit breaker whereby the fixed and movable contacts in one phase become electrodynamically separated before the operating mechanism responds to separate the fixed and movable contacts within the remaining phases, requires the use of contact springs to hold the fixed and movable contacts against electrodynamic forces induced by momentary current surges. The movable contact arm is reverse-biased to a closed position by means of a pair of powerful contact springs which hold the movable contact attached to the end of the movable contact arm tightly against the fixed contact assembled on the bottom of the circuit breaker case. When the circuit breaker operating handle manually drives the movable contact arm and the attached movable contact from the open position distal the fixed contact, to the closed position against the fixed contact, the reverse bias of the spring force against the movable contact arm must first be overcome in order for the operating mechanism springs to over-center and drive the movable contact arm to its open and closed positions. The powerful contact springs necessitate the corresponding use of a pair of even more powerful operating mechanism springs to overcome the contact spring reverse bias during the manual opening and closing operation and to provide an over-center "OFF" position.
One purpose of the invention accordingly is to provide means for overcoming the reverse bias of the circuit breaker contact springs during the manual contact closing operation without requiring operating mechanism springs of increased strength.